The present invention relates to switching-mode power supplies and, more particularly, to primary-side start circuits for switching-mode power supplies.
Switching-mode power supplies for the efficient regulation of a direct-current (DC) voltage are well known. A switching-mode power supply "chops" a DC input voltage to produce a high-frequency alternating current (AC) rectangular waveform by operating a power transistor in a non-linear "switching" mode. A pulse width modulator (PWM) controller generates a pulse train that is used to switch the power transistor on and off. The resulting high-frequency AC rectangular waveform is rectified and L-C filtered to produce an output DC voltage. The DC output voltage, which varies with the width of the switching pulse, is compared against a reference voltage to produce a DC error signal. The PWM controller uses the error signal to adjust or modulate the switching pulse width and thereby regulate the DC output voltage.
Many applications require isolation between the power source and the load, and a switching mode power supply typically includes a power transformer that, inter alia, provides isolation. The PWM controller may be placed either on the primary (i.e., source) side or secondary (i.e., load) side of the transformer. When the PWM controller is placed on the primary side it can be easily powered by the primary power source, but feedback of the low-level analog error signal across the isolation boundary is difficult. A secondary-side PWM controller offers several advantages over a primary-side PWM controller, including easier closure of the analog feedback loop, easier feedback of the high-level digital PWM signal across the isolation boundary, simpler load management (e.g., power monitoring of the load, sequencing from the load, frequency synchronization with the load), easier load current sensing, a load sharing capability, faster loop response and the direct drive of synchronous rectifiers. However, the secondary-side PWM controller, which is isolated from the primary power source, operates from secondary-side power and thus cannot generate a PWM signal to initiate operation of the power transistor switch.
One solution to the secondary-side PWM controller problem is to provide a primary-side startup circuit, typically comprising a free running oscillator, to generate a switching pulse train and thereby initiate operation of the power transistor. For example, U.S. Pat. No 4,694,384 (Steigerwald et al.) discloses a switching-mode power supply having a primary-side voltage controlled oscillator (VCO) for generating a startup switching pulse train and a primary-side digital data detector for receiving digital error data from a secondary side PWM generator via an electrically-isolated data transmission device, such as a pulse transformer. A multiplexing circuit selects the VCO output during startup and switches to the PWM generator output during normal "run"operation. U.S. Pat. No 4,887,199 (Whittle) discloses a switching-mode power supply having a primary-side relaxation oscillator for generating a startup switching pulse train and a circuit for disabling the startup switching pulse train when a secondary-side pulse generator begins normal operation. In both patents, the passing of PWM control to the secondary-side for normal operation is absolute with only secondary-side information being factored into the PWM control signal. The inability to make primary-side adjustments to the PWM control signal renders the isolated switching-mode power supply a less than optimal solution for a typical universal-input power supply.